The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Vehicle engines generate power for tractive torque and a plurality of extraneous devices such as an alternator, an A/C compressor and a power steering pump. When these extraneous devices demand power, the engine increases output power to maintain tractive power, which is noticeable when the engine is idling. Likewise, when these extraneous devices no longer demand power, the engine decreases output power. Output power provided by a vehicle engine is often controlled during idle by an electronic throttle control device with limited control authority to make minor adjustments to the intake airflow. To address this limitation, a separate idle air control valve has been used to achieve small adjustments to the intake airflow and therefore the engine idle speed and power output.
Known idle speed control systems for spark-ignition engines use spark timing to adjust engine power and maintain engine idle speed in response to changes in engine load. Spark timing can be controlled to manage engine torque and control engine speed. By advancing or retarding spark timing the engine power can be changed. Control of spark timing provides a fast acting torque control mechanism. Mean best torque (MBT) spark timing provides maximum engine torque at fixed speed, mixture composition and flow rate. An engine can be calibrated at a nominal spark timing that is retarded from the MBT spark timing, allowing the engine to rapidly adjust torque output by advancing or retarding spark timing to accommodate load transients. Thus, nominal spark timing for engine control during idle differs from MBT spark timing to allow spark timing adjustments that can either increase or decrease engine power during idle. Engine operation at the nominal, non-MBT spark timing burns fuel without a corresponding increase in engine power. Such engine operation generates a lower output torque and inefficiently consumes fuel.